1. Field of the Invention
The present invention relates to an irradiation apparatus for irradiating particle beams or corpuscular rays to a location to be irradiated as well as an irradiation method using this apparatus.
2. Description of the Related Art
Known methods of enlarging the irradiation field of an irradiation apparatus, which treats cancer by using such a kind of particle beams, generally include a double scatterer method and a Wobbler method. In the double scatterer method, there are arranged two scatterers through which a particle beam passes so that a uniform dose distribution can be formed. On the other hand, in the Wobbler method, a particle beam is irradiated on a scatterer while being moved in a circle by means of an electromagnet so that a uniform dose distribution can be formed in the vicinity of the center of the circle. The field of irradiation thus obtained is usually of an area of from about 15 cm×15 cm to about 20 cm×20 cm, which is sufficient for irradiation of a spot or location to be treated in a lot of cases (for instance, see a patent document: Japanese patent application laid-open No.H10-151211).
However, there may also be some cases requiring irradiation fields larger than the above-mentioned one. For instance, larger flows are often required for elongated areas such as the oesophagus, the neck of the womb (cervix), and the area from the mandible to the shoulder. In these cases, the shape of a large irradiation field as required is not a square or a circle but a rectangle or an oval of from 15 cm×20 cm to 20 cm×25 cm or larger.
One method of achieving such a large irradiation field, conceives that the distance from an irradiation field enlarging device to a location to be irradiated (hereinafter also referred to as the target) should be increased. However, in this case the rotating gantry used is a heavy structure having a diameter of about 10 m and a weight of about 200 tons, and at the same time it is a precision machine with its center of rotation designed to keep an accuracy of about +1/−1 mm. Thus, it is difficult to further enlarge such a structure even in terms of cost as well as accuracy.
In addition, another method of achieving a large irradiation field is to strengthen the performance of an irradiation field enlarging device. In the Wobbler electromagnet, however, there arises a problem that when the magnetic field strength is increased to generate an alternating magnetic field, ac loss due to the eddy current generated in the iron core of the magnet becomes large, thereby heating the iron core to a high temperature.
Moreover, although a strategy of lengthening the magnetic poles of the electromagnet can be conceived, too, it is undesirable from the viewpoint of keeping the rotating gantry small.
On the other hand, in the case of using the double scatterer method, there is a technique of increasing the thickness of the scatterer to enlarge the irradiation field. In this case, however, beams passing through the scatterer are decelerated therein, so increasing the thickness of the scatterer shortens the range of the beams inside the target. Therefore, there is also a limit to the thickness of the double scatterer to be used. Thus, in order to increase the thickness of the double scatterer and to ensure the beam range inside the scatterer at the same time, it is inevitable to raise the beam energy, thus resulting in a larger particle accelerator.